Powder feeding using a conveying hose is typically done using flight conveying in which particles are entrained in a carrier gas stream to deliver the powder to an end process such as a spray gun. Instabilities in the flow can develop for a number of reasons resulting in fluctuations of the powder flow that can affect the spraying results.
Lasers have been employed to measure the light transmission through the powder flow stream. However, this method suffers from several drawbacks:                1. The hose conveying the powder stream has to be altered to provide a suitable location, preferably near the process or end use, where the laser can shine through the powder flow. This alteration on its own can introduce instability by creating discontinuities in the flow.        2. The signal attenuation for the laser light transmission is high and, although this provides excellent sensitivity, the signal can quickly become saturated under high flow conditions, preventing proper diagnosis. This is especially the case with high mass flow ratios of powder to carrier gas, in which the powder flow can completely block the laser light transmission.        3. Adding the laser adds considerable cost to the process while also adding complexity to an already complex system that must accurately feed and control powder flow.        
The two primary types of powder feeders are fluidic and volumetric. Both types can feed a wide variety of powders ranging in size from about 150 μm to less than 5 μm with powder densities from about 3 g/cc to as high as 15 g/cc. Additionally these feeders can feed powders at a wide range of feed rates from about 1 g/min to as high as 300 g/min. These powder feeders can be utilized in industrial applications in, e.g., pharmaceutical, food processing, thermal spraying, and other suitable industries.
A number of powder feeders incorporate a pressure transducer in the powder line at the exit of the powder feeder, e.g., Oerlikon Metco 9MP-CL. The transducer is used to define a hose pressure feedback that is in turn used to calculate the hopper differential pressure required to control the feed rate in a fluidic feeder using a gravimetric setup. The hose back pressure in some versions of powder feeders can also be used as a safety detection to isolate the powder hopper under sudden or unexpected high back pressure conditions. This signal is also displayed on the powder feeder as a heavily filtered signal, which is updated about once per second. To date no feeder has attempted to use the pressure signal to determine whether the powder flow in the hose is stable.